Induction heating coil



, Aug. 7, 1945.. A Y R. M. BAKER ETAL t 2,381,245

" 4 n INDUCTION HEATING COIL Filed oct. 31., 1942 `\\\\\\\\\\\\\\\|s\ SLATTORNEY Patented Aug. 7, 1945 INDUCTION HEATING COIL Robert M. Baker,Harold G. Frostick, and Charles C. Whittaker, Pittsburgh, Pa.,assgnorsto Westinghouse Electric Corporation, East Pittshurgh, Pa., acorporation of Pennsylvania Application Octoberl 31, 1942, Serial No.464,041

Claims.

Our Vinvention relates to inductive heatingcoils, and particularly tosuch coils which are utilized to heat a rapidly moving, long, thin,flat, metal strip, or one or more wires, or other elongated member whichis moving axially through the heating coil. In many inductiveheating-coils of this nature, where very high power must be putinto thecoil, the voltage of the coil may be a distinct limitation as to itsdesign, and particularly the maximum voltage appearing between thecoil-terminals and the metal strip or other work to be heated, withinthe coil. Y

While our invention is not limited to any particular application, thespecial design of coil which we have introduced appears to liind itsmost important present application in the tinplate-flowing apparatuswhich is described and claimed in an application of Glenn E. Stoltz andRobert M. Baker, Serial No. 464,040, led October 31, 1942, and assignedto Westinghouse Electric & Manufacturing Company.

In this application of our invention, a milelong, thin, iiat,electrolytically tin-plated steel strip, having a gauge usually of theorder of 0.008 to 0.011 inch, and having a width of from 28 to 36inches, is passed at a speed of the order of eleven or twelve miles perhour through an inductive heating-coil, which is required to heat thestrip some 300 to 400 F. within as short a distance as possible, or atleast within a distance which is within a reasonable factorof safety ofthe distance which would produce buckling of the strip as a result ofthe strains therein, from thermal expansion, exceeding the elastic limitor yield point of the strip. This high rate of heatinput into the stripis needed in order to make it possible to heat-treat the plated strip inthe same line or manufacturing process in which the strip iselectroplated, dried,kheated, quenched, cleaned, and otherwise treated.

The heating is for the purpose of raising the tinplate to, or slightlyabove, its melting point, which is 452 F., so that the surface-tensionof the molten tin will cause it to flow evenly over the steel, producinga solid, non-spongy, uniform texture, and making it possible to obtainas good -a corrosion-resistant, shiny coating, with a tinplate only30millionths of an inch thick, as was previously obtained with a tinplateof three times that thickness, as applied by the old hot-dip procf ess.'I'he high concentration of heat, along a relatively small length` ofthe strip, is needed in order to make it possible to heat-treat thestrip in the same line in which it had been electroplated, so`thatseparate manufacturing operations will not be needed; and the use ofinduction heating is necessary in order to be able to get enough heatinto the strip in a small space, to make the strip-heating substantiallyindependent of slight variations in gauge, to avoidl physical contactwith the easily marred tin surface, and to provide the high-speedheat-input control which is necessary for satisfactory operation onthese high-speed tinning lines.

The object of our present invention is to provide a practicalcoil-design and construction, which is capable of serving in this newand im- `portant field of heating long tinned strips, orl

the like, with very large amounts of radio-frequency power, I

A more specific object of our invention is toprovidek an improvedembedded shield-construction which is associated with an open-ended, boxor liner which is disposed within the inductive heating coil, andthrough which the strip to be heated passes, said box or linercomprising solid molded insulation having a grounded shield embeddedtherein.

A more specific object of our invention is to provide a novel form ofshield, for such a highfrequency, high-iiux inductive heating-coilliner,

said shield being in the form, of a plurality of fine non-magneticparallel spaced wires running in an axial direction, and havinggroundingmeans joining the wires at one or both ends, at a point outaxially beyond the end of the coil,

so as to be out of the lntensest4 portion of the magnetic eld ofthecoil, said shield-wires having such a small diameter that they are notheated to any material extent at the frequencies and flux-densitieswhich are utilized to heat the strip, particularly where the strip ismade of steel or other magnetizable material.

A further object of our invention is. to provide a flat, open-endedbox-like insulating liner for such an inductive heating coil, the linerhaving an opening therethrough through which the strip to be heatedpasses, the two sides of the liner being flat, parallel, and closelyspaced from the flat surfaces of the strip, and the two ends of theliner being spaced -a greater distance from the edges of the strip, thetwo'flat sides of the liner having grounded shielding-means associatedtherewith, extending axially beyond the axial extent of the coil, andextending transversely beyond the'width of the strip, so as toadequately shield the edges of the strip.

A still further object of our invention is to provide a composite linerfor such a coil, said liner comprising a relatively permanently securedouter box-portion, and an inner slot-liner removably secured in positionalong the inner sur- -face of each of the two at sides of the box.

A still further object of our invention is to provide an inductiveheating coil having a shielded dow and the window-framed making contactwith the shielding-means of the liner.

With the foregoing and other objects in view, our invention consists inthe combinations, structures, parts, systems, and methods'hereinafterdescribed and claimed, and illustrated in the accompanying drawingwherein:

Figure l is a somewhat diagrammatic horizontal or transverse sectionalview through a coilconstruction illustrating the principles of ourinvention,

Fig. 2 is a front elevational View thereof, with parts 'broken away toillustrate the construction,

Fig. 3 is a central longitudinal or vertical sectional view thereof, and

Fig 4 is an enlarged fragmentary detail sectional-view showing theembedding of the ilne wires of the shielding-means in the moldedinsulating box-side of the liner.

As shown in Fig. 2i, our invention is illustrated as being applied tothe heat-treatment of a rapidly moving tinned steel strip I which israpidly moving downwardly through an inductive heating-coll structurewhich is indicated, in its entirety, by the numeral 2. The coil-properis indicated at 3, and comprises a plurality of turns of a hollowconductor, wound in a single layer, and having water or other coolantcirculating therethrough, as indicated by the arrows 4, successive turnsof the coil being separated by annular insulating blocks 5. High-powerhigh-frequency energy is supplied to the coil 3 from anoscillation-generator which is diagrammatically indicated at 6 in Fig.2.

Within the coil 3, there is disposed an openended box-like linerl ofstrong hard solid insulating material, said box or liner l having anopening 8 therethrough, through which the strip I passes. The box orliner l hasl two liat parallel box-sides 9 which are closely spaced fromthe flat surfaces of the strip, and the narrow ends of the box areclosed by two box-ends II which are spaced a greater distance from theedges I2 of the strip, so that the box-sides 9 are considerably widerthan the strip I. The box 'I extends, in a vertical or axial direction,considerably greater distance than the axial extent of the coil 3, sothat the upper and lower ends of the box or liner 'I extend into aregion where the axial magnetic field of the coil is not very strong, ornot as strong as within the coil-proper.

Because of the large power-input into the coil 3, the` voltages involvedare quite high, so that, unless suitable shielding-means is provided,the high electrostatic voltage-gradients will produce excessive coronaor space-current discharges, radiating out from the surfaces or edges ofthe strip I.

An important feature of our invention is to provide a suitable groundedelectrostatic shield which is economically feasible, and practicallyoperative, to guard the strip I against spacecurrent discharges orcorona, while offering slight hindrance to the axial magnetic lines offorce flowing in the strip, which is another way of saying that theshielding itself does not consume much of the magnetic energy of thecoil, and is not itself heated to any considerable extent. ln fact, theavoidance of excessive heating of the shielding-means is as important anobject as the cutting down of the strength of the alternating magneticflux flowing axially or longitudinally in the strip I. l

'I'he shielding-means which we utilize is an embedded shield; and it isa shield composed of a dividual wire plurality of parallel nenon-magnetic wires running axially and having such small gauge that theyare not materially or excessively heated at the frequency andfield-strength which is utilized in the coil 3. Moreover, our shield isa shielding-means which is associated with the dat box-sides 9, andwhich extends laterally well beyond the edges I2 of the strip I, so asto adequately shield these edges, without the necessity for extendingthe shielding-means completely around the periphery of the liner l.

While, in the broadest aspects of our invention, we are not limited toany particular kind of shield, we have illustrated a very much preferredform of embodiment of the shield which, all things considered, seems tous at present to be far superior to any other available form ofshielding-means for this high-frequency inductionheating application.

As shown, our shielding-means comprises o. large number of closelyspaced parallel hon-magnetic wires I5 associated with each ol theboxsides 9, and running in a longitudinal direction, that is, parallelto the axis of the coil il. The wires I5 thus run from substantially thetop to substantially the bottom oi' each box-side 9. These wires are ofsuch small diameter that they are not significantly heated by thealternating flux flowing lengthwise therethrough, at the fluxdensity andthe frequency involved. Copper Wire of 0.005 inch diameter has proven tobe very satisfactory, in one form of embodiment of our invention,although we are not limited, oi course, to this size or material. Suchiine wires are extremely ne and hard to manage, and they are applied inany convenient way which holds the 4wires in parallel strands, whilebeing applied to the box-side 9 in the process of manufacture.

The box-side 9 is preferably made of a reinforced molded solidinsulating material, preferably made up of a number of layers of glasscloth, impregnated with a moldable resin or heatand-pressure-hardenedinsulating composition. In the process of manufacture, when nearlyenough layers of impregnated glass cloth have been piled up to make abox-side 9, the plurality of closely spaced parallel wires I5 are thenlaid over the top of the partially prepared box-side, with a narrowcross-strip oi thin copper foil l5, or other extremely thinconducting-means or high-resistance conducting-means, laid across theentire group of wires, preferably a strip or foil I6 near each end ofthe wires, with a length of strip I5 extending over for making agroundconnection as indicated at Il in Fig. 2. The foil or strip IB ispreferably soldered to each in- I5. After this is done, with the wireslaid in place on the partially completed box-side 9, as shown in Fig. 4,one or more additional glass-cloth layers I8 are added and cemented intoplace, preferably by subjecting the entire material to heat and pressurei'or setting the moldable insulating binding-'naterial with which theglass cloth is impregnated. The result is a hard stiff stronginsulating-board, with the wires l5 and their end-connected buses orfoils I G embedded within the solid molded material of theinsulating-board 9.

Two boards 9, preferably prepared in the manner described, are assembledwith the insulating end-pieces II to make the box or liner l, thebox-sides 9 being assembled with the parallel-wire shield I5 disposednear the inner surface. that is, the surface closest to the strip I tobe heated, and farthest away from the induction heating coil 3 whichsurrounds the box 1. Since the shield I5-l6 is grounded (preferably atonly one end), practically all of the voltage-drop between the cpil 3and ground appears between the coil and the shield I5-I6, rather thanbetween the shield and the strip I being heated. Since the box-side 9 iswider than the strip, and since the assembly of parallel shielding-wiresI5 extends out over practically the whole width of the box-side, so as'to extend the shield transversely considerably beyond the edges of thestrip I, said strip I is effectively shielded from excessiveelectrostatic field-gradients, thus eifectually preventing co rona fromappearing on the stripl I.

In order to ,prevent corona from appearing on the coil 3, between thecoil 3 and the outer portions of the box or liner 1, it i's quitedesirable for the coil to be immersed in solid insulating material whichexcludes any gaseous medium, because corona is a gaseous-ionizationphenomenon, which cannot take place in a liquid or solid insulatingmedium. The shield I5, being an embedded shield, as described, isalsoimmersed in solid insulating material which substantially excludesany gaseous medium, thus preventing the formation of corona on theshield, under the operating-conditions of the device. For solidlyembedding the coil 3, we prefer to utilize a hardenable or moldableinsulating material for filling all the spaces around the box or liner 1and between said liner and the coil 3 and preferably outside .of thecoil.

As a convenientmeans for providing a suitable support for the coil 3 andthe liner 1, and for at the same time' providing a suitable mold for themoldable insulating-material which is to be flowed around the coil 3 andhardened in place, we preferably provide an outer box surrounding theinner box or liner 1 and the coil 3, with a suitable space in between,and we closethe space between the two boxes 1 and'20 with abottom-member 2i so that we can pour a hardenable insulating resin orcement into place in such manner as to substantially fill the spacebetween the two boxes, as indicated at 22, in such manner as tosubstantially exclude Aair or any gaseous medium, and to eifectuallyembed the hollow conductors 0f the coil 3.

vides an extremely strong and rigid support for the coil 3, as well asfor the inner box or liner 1.

In s ome cases, as set forth in the previously mentioned Stoltz-Bakerapplication, it is desirable to provide the coil-assembly 2 with awindow or opening 23, which is provided in one of the box-sides 9, andin the corresponding side of the outer-frame, 20, with a window-frame 24disposed therebetween, so that the windowframe 24 extends outwardly fromthe window 23 in the inner box-side 9, in a direction away Vfrom thestrip I being heated. In accordance with our present invention, the twowindows 23, and

, the inner surface of the window-frame 24 are painted all around with asemi-conducting adherent coating 25, which is necessarily shownexaggerated in thickness, in Figs. l and 3. This coating makeselectrical contact with the exposed ends of the wires I5, which are cutwhen the window 23 is cut in the shielded box-side 9, so that thesemi-conducting coating 25 is grounded to the .grounded shielding-wiresI5. The grounded semi-conducting adherent coating 25 may be an aquadagcoating, or any other semi conducting adherent coating, such as a com-Whenthis insulating material sets Orhardens, it propound of varnish, orother insulating binder, with a semi-conducting powdered material suchas wood charcoal or powdered titanium dioxide which has been partiallyreducedA in a hydrogen furnace.

As explained in the Stoltz-Baker application, the window 23 is utilizedto accommodate light-4 sensitive scanning-means (not shown) fordetermining and fixing the location of the flowline -on the strip I,which is the line 0f devmarcation between the unmelted tin-platedsurface having a matte surface, as it comes from the electroplatingtanks (not shown), and the same tin-plate surface after it has beenmelted so that its surface-tension causes it to have the familiar brightmetallic polished surface which is familiar in our so-called tin" cans.

Our grounded semi-conducting coating 25 on the window 23 effectuallycooperates with the as indicated in Fig. 3. As shown, this rubbergrounded parallel-.wire shield I5 in preventing corona at any point.

It will be noted that the cross-connecting foils or strips I6 which jointhe shielding-wires I5 at' their upper and lower ends (or at eithertheir upper end or their lower end, in the case of boxsides having nowindow) are disposed respectively above and below the axial extent ofthe coil 3, so that they are disposed out 0f the region of the strongestmagnetic eld cf the coil 3, -thus reducing the heating of the end-strips or foils I6.

It is also a feature of our invention to make up the liner 1 in twoparts, orto place an inner slotliner 29 in position along the innersurface of l each of the two flat box-sides 9 of the previouslydescribed liner-,member 1. The inner liner 29 is made of any suitablegood-wearing insulating material which may be the same las the material'of the permanent liner 1, but may also be a different material such as'asbestos-board. This inner liner 29 is securely and removably insertedvwithin the permanent liner 1, as by means of screws 3i] (Fig. 3) at thetop of the apparatus,

This removable inner liner 29 is provided because the liner of theinduction-coil assembly is the part which is the most likely to fail, orto wear out, if there is any failure. Damage may be done to the liner,for example, if the tinned Istrip I which is being treated should break,when running at a high speed; or, if the strip I should frequentlyoperate under conditions of insuflicient tension on the strip, it mightflop over and rub against the inner liner, to the detriment both of theliner and the surface-finish on the strip. As an additional detail, wehave also provided a soft rubber buffer or guard 3l at the top orentrance-end of the hole 8 through the liner,

be encountered if the strip came into contact with the hardliner-material.

Near the bottom of the induction-coil assembly 2, where the heat is thegreatest because of radiation from the hot strip I, we provide awatercooled coil 21 which is embedded in the insulating cement 22. Oneend of the coil 21 is insulated,

as by means of a rubber-hose connection 28, to

avoid short-circuiting the coil.

It is believed that the operation of our appa.-y

ratus will be apparent from the explanations which have been given alongwith the descrippose of illustrating the essential features of ourinvention, without encumbering the illustration with numerousconstructional details which have` no real part of the invention, wewish it to be understood that such illustration is only illustrative,and that our invention is susceptible of embodiment in a number ofdifferent forms. We desire, therefore, that the appended claims shall beaccorded the broadest construction consistent with their language.

We claim as our invention:

1. Inductive heating apparatus for heating a long thin fiat strip ofconducting material which is continuously moving in the direction of itslength, said apparatus comprising an open-ended box of strong solidinsulating material having an opening therethrough. through which thestrip may pass, the two sides of the box being fiat, parallel, andadapted to be closely spaced from the flat surfaces of the strip, thetwo ends of the box being adapted to be spaced a, greater distance fromthe edges of the strip, an inductive heating-coil, the box beingdisposed within the induc- `tive heating-coil and extending at each endaxially beyond the end of the coil, and grounded electrostaticfield-shielding means associated with each of the two fiat sides of thebox for guarding the strip against space-current discharges whileoiiering but slight hindrance to the axial magnetic lines of forcewithin the strip, said shielding-means extending axially beyond theaxial extent of the coil and adapted to extend transversely beyond thewidth of the strip.

2. Inductive heating apparatus for heating a long thin fiat strip ofconducting material which is continuously moving in the direction of itslength, said apparatus, comprising an open-ended box of strong solidinsulating material having an opening therethrough through which thestri-p may pass, the two sides of the box being flat, parallel, andadapted to be closely spaced from the fiat surfaces of the strip, thetwo ends of the box being adapted to be spaced a greater distance fromthe edges of the strip, and an inductive heating-coil, the boxbeing'disposed within the inductive heating-coil and extending at eachend axially beyond the end of the coil, the two fiat sides of the boxcomprising solid molded insulation having a grounded conducting shieldembedded therein so as to be immersed in solid insulating material whichsubstantially excludes any gaseous medium from the embedded shield.

3.,Inductive heating apparatus for heating a. long thin fiat stri-p of amagnetizable metal which is continuously moving `in the direction of itslength, said apparatus comprising an` open-ended non-magnetic parallelspaced conducting wires running in an axial direction along each of thetwo fiat sides of the box across a width which is adapted to be greaterthan the width of the strip, and extending axially beyond an end of thecoil, and grounding-means joining the wires at an end, out of theintensest portion of the magnetic field of the coil.

4. Inductive heating apparatus for heating a long thin flat strip ofconducting material which is continuously moving in the direction of itslength, said apparatus comprising an open-ended box of strong solidinsulating material having an opening therethrough through which thestrip may pass, the two sides of the box being flat, parallel, andadapted to be closely spaced from the fiat surfaces of the strip, thetwo ends of the box being adapted to be spaced from the edges of thestrip, and an inductive heating-coil, the box being disposed within theinductive heatingcoil and extending at each end axially beyond the endof the coil, said box comprising two portions comprising a relativelypermanently secured outer box-portion, and an inner slot-liner removablysecured in position along the inner surface of each of the two flatsides of the box.

5. The invention as defined in claim 1, characterized by a fiat side ofthe box having a window therein, a window-frame surrounding the windowand adapted to extend outwardly away from the strip, and a. groundedsemi-conducting coating on the surfaces of the window and thewindow-frame making contact with the shielding-means of the box-side.

6. The invention as defined in claim 2, characterized by a fiat side ofthe box having a window therein, a window-frame surrounding the windowand adapted to extend outwardly away from the strip, and a groundedsemi-conducting coating on the surfaces of the window and thewindow-frame making contact with the grounded shield of the box-side.

, "1. The invention as defined in claim 3, characterized by a fiat sideof the box having a window therein, a window-frame surrounding thewindow and adapted to extend outwardly away from the strip. and agrounded semi-conducting i coating on the surfaces of the window and thewindow-frame making contact with the fine wires of the box-side.

8. The invention as defined in claim 1, in combination with a solid massof insulating material in which the coil is embedded and filling thespace. to the substantial exclusion of any gaseous medium, between thecoil and the box through which the strip may pass.

9. The invention as defined in claim 2., in combination with a solidmass of insulating material in which the coil is embedded and fillingthe space, to the substantial exclusion of any gaseous medium, betweenthe coil and the box through which the strip may pass.

10. The invention as defined in claim 3, in combination with a solidmass of insulating material in which the coil is embedded and fillingthe space, to the substantial exclusion of any gaseous medium, betweenthe coll and the box through which the strip may pass.

ROBERT M. BAKER. HAROLD (3. FROSTICK. CHARLES C. WHITTAKER.

